The present invention relates generally to semiconductor processing and fabrication methods, and more particularly, to a capacitive vertical silicon bulk acoustic resonator.
High frequency mechanical resonators such as surface acoustic wave (SAW) and film bulk acoustic wave resonators (FBAR) and filters are widely used in RF front-end duplexers as band-select filters. Such resonators are discussed in Masanori Ueda, et al., “Ultra-miniaturized and high performance PCS SAW duplexer with steep cut-off filters”, MTT-S 2004, Vol. 2, pp. 913–916, and R. C. Ruby, et al, “Thin film bulk wave acoustic resonators (FBAR) for wireless applications”, Ultrasonics Symposium, 2001, Vol. 1, pp. 813–821, respectively.
Recent developments in silicon micromachining technologies have paved the way for implementation of high frequency silicon capacitive resonators with close to one order of magnitude higher quality factors (Q) compared to the SAW resonators and film bulk acoustic wave resonators. Such devices are gradually entering the market, opening up new opportunities for more advanced wireless communication systems. Due to their lithographically defined resonant frequencies, in-plane capacitive resonators with operating frequencies in a wide range from tens of kHz up to GHz can be implemented on the same substrate simultaneously. This will allow implementation of multi-band wireless communication systems.
In addition, due to their much larger Q values compared to piezoelectric resonators, capacitive resonators may enable direct channel selection right after the antenna resulting in dramatic simplification of the transceiver architectures. Low cost batch fabrication using conventional silicon processing techniques is another advantage for silicon resonators.
Despite resonant frequencies in the VHF and UHF range with quality factors as high as a few tens of thousands, high equivalent electrical impedance remains the major drawback for incorporation of such devices in electronic systems. Fabrication and mechanical design techniques have been utilized to alleviate the impedance issue, but the demonstrated impedances in the VHF and UHF range have heretofore not been promising. See for example, S. Li, et al, “Micromechanical hollow disk ring resonators”, MEMS 2004, pp. 821–824, S. Pourkamali and F. Ayazi, “SOI-based HF and VHF single-crystal silicon resonators with sub-100 nm nanometer vertical capacitive gaps”, Transducers '03, pp. 837–840, S. Pourkamali, et al, “VHF single crystal silicon capacitive elliptic bulk-mode disk resonators part II: implementation and characterization”, JMEMS, Vol. 13, No. 6, December 2004, and S. Pourkamali and F. Ayazi, “High frequency capacitive micromechanical resonators with reduced motional resistance using the HARPSS technology”, Proceedings, 5th Silicon RF topical meeting 2004, pp. 147–150.
The S. Pourkamali et al. paper “High frequency capacitive micromechanical resonators with reduced motional resistance using the HARPSS technology”, Proceedings, 5th Silicon RF topical meeting 2004, pp. 147–150, discusses disk resonators that are somewhat similar in function to the capacitive vertical silicon bulk acoustic resonator disclosed herein. However, nothing is disclosed or suggested in this paper regarding a resonator element having a length-to-frequency-determining-width ratio larger than one, or a resonator element that comprises a bar having rectangular cross sections, for example.
A paper entitled “High-Q Single Crystal Silicon HARPSS Capacitive Beam Resonators With Self-Aligned Sub-100-nm Transduction Gaps” by Siavash Pourkamali et al., published in Journal of Microelectromechanical Systems, Vol. 12, No. 4, August 2003, discusses capacitive beam resonators that are somewhat similar to the capacitive vertical silicon bulk acoustic resonator disclosed herein. However, nothing is disclosed or suggested in this paper regarding a resonator element that is coupled by way of support structures to bias pads, or that the resonant frequency of the resonator element is primarily determined by a single in-plane lateral dimension, for example.
It would be desirable to have capacitive vertical silicon bulk acoustic resonators that operate in the HF/VHF/UHF bands, exhibit low impedance values and have quality factors that are as high as possible.